Magnetic resonance imaging is an imaging technique used to visualize the inside of an object (or subject) under study (e.g., a human or animal body or a body part or an entire laboratory animal or specimen from the animal or a plastic test phantom). MRI relies on the relaxation properties of excited hydrogen nuclei in water and fat. When the object to be imaged is placed in a powerful, uniform magnetic field, the spins of the atomic nuclei with non-zero spin numbers (essentially, an unpaired proton or neutron) within the tissue all align in one of two opposite directions: parallel to the magnetic field or antiparallel. Magnetic field strengths for MRI studies of animals typically require 4.7 T, and magnets up to 17 T have been reported. For a comparison, the average magnetic field of the Earth is around 50 μT (or 0.5 G).
Single photon emission computed tomography (SPECT) is a nuclear medicine tomographic imaging technique using gamma-rays. Conventionally, this imaging technique accumulates counts of gamma photons that are absorbed by a scintillator crystal. The crystal scintillates in response to interaction with gamma radiation to produce a flash of light. Photomultiplier tubes (PMTs) behind the scintillator crystal detect the flashes of light and a computer sums the fluorescent counts. The sum of fluorescent counts is a measure of the energy of an individual detected gamma-ray, and the location of the detected gamma-ray is computed from the distribution of the fluorescent counts among several PMTs. The computer in turn constructs an image of the relative spatial density of gamma-ray counts, accumulated as a series of detected gamma-rays whose measured energy is within a range that is selected by the operator, and displays the image on a computer monitor. This image then reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged.
Although there may be benefits to combine SPECT and MRI, any theoretical benefits of trying to combine SPECT and MRI within a single system have been mostly dismissed because the functions of the PMTs in a typical SPECT system are severely compromised by the high magnetic fields needed for MRI and because magnetic field uniformity needed for MRI is distorted by the PMTs (i.e., the ferro-magnets in the PMTs).
Recent advances in semiconductor technology have opened the possibility of replacing the PMTs and the scintillator crystal of a SPECT system with a semiconductor detector, such as a cadmium zinc telluride (CdZnTe or CZT) detector. The CZT detector may operate in the magnetic field inside an MR imaging apparatus. The CZT detector is referred to as a direct detector of radiation and operates by producing negative and positive charges (or electrons and holes) through interaction with gamma photons. However, combining a CZT detector for detecting gamma photons is still not a trivial task because the electrons and holes of the CZT detector need to travel non-negligible distances to generate their signals (e.g., travel distances of 2-5 mm and even larger). This presents possible Lorentz-force effects where signal generation may be distorted.
In addition, it may be necessary to remove the electronics for signal amplification, address generation, logical operations, and other processing functions from the CZT module (in the high magnetic field) and to bring these electronics to a more distant location (in which a lower field can be found), thereby removing a cause of interference (e.g., either the offending electronics does not function in the high field or the offending electronics causes the MRI to have artifacts). However, the electronics located away from the magnetic field need to be connected via relatively long cables that result in an increased signal noise and distortion.
In view of the foregoing and as discussed in Wagenaar et al. “Rational for the Combination of Nuclear Medicine with Magnetic Resonance for Pre-clinical Imaging,” Technology in Cancer Research and Treatment, ISSN 1533-0346, 2006, Vol. 5, No. 4, pp. 343-350, which is incorporated by reference herein in its entirety, it would be desirable to combine MRI with single photon nuclear imaging, such as SPECT, to provide a more complete coverage between high resolution, anatomical imaging, and genetically targeted molecular imaging that overcomes the detrimental effect of the magnetic fields produced by the MRI.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.